A string is the programming equivalent of text - it is a string of text characters.
A string is created by using single or double quotation marks:
'Hello, World'
"Hello, World"## ans = Hello, World
## ans = Hello, WorldA number can also form part or all of a string - just use the quotation marks to create it as above:
% Create a string that happens to contain a number
'123'## ans = 123Make sure you don’t get confused between numbers that are strings and numbers that are scalars (actual numerical values)!
% This number is a string
st = '123';
typeinfo(st)
% This number is a scalar
no = 123;
typeinfo(no)## ans = sq_string
## ans = scalarThe length() function will give you the length of your string:
st = 'Hello, World';
length = length(st)## length = 12Just like vectors, you can get the n-th character from a string by using round brackets:
st = 'Hello, World';
first_letter = st(1)## first_letter = HThe command end will get you the last element:
st = 'Hello, World';
last_letter = st(end)## last_letter = dA range will return the characters corresponding to the numbers in the range:
st = 'Hello, World';
range = 1 : 2 : length(st);
odd_letters = st(range)## odd_letters = Hlo olFind the first occurrence of a sub-string:
st = 'Hello, World';
idx_left = index(st, 'l')## idx_left = 3Find the last occurrence of a sub-string:
st = 'Hello, World';
idx_right = rindex(st, 'l')## idx_right = 11Find all occurrences of a sub-string:
st = 'Hello, World';
idx = find(st == 'l')## idx =
##
## 3 4 11The strfind() function does the same thing:
st = 'Hello, World';
idx = strfind(st, 'l')## idx =
##
## 3 4 11Note that all the above are case sensitive: lowercase l is different to uppercase L:
st = 'Hello, WORLD';
idx = strfind(st, 'l')## idx =
##
## 3 4Are two strings the same? The strcmp() function will return 1 for ‘yes’ and 0 for ‘no’:
strcmp('Hello', 'World')## ans = 0This is different to the == comparison we used in the ‘Finding’ section above. This compares each character in two strings:
'Hello' == 'Hxlxo'## ans =
##
## 1 0 1 0 1Strings can be joined together by putting them inside square brackets, separated by spaces:
greeting = 'Hello';
name = 'World';
% Join four strings together
[greeting ', ' name '!']## ans = Hello, World!We saw earlier that there is a difference between numbers that are strings and numbers that are scalars. The function num2str() will convert a number into a string and, unsurprisingly, str2num() will convert a string into a number:
% Create a scalar
sc = 12345;
% Convert the scalar into a string
st = num2str(sc);
typeinfo(st)
% Convert back into a scalar
sc = str2num(st);
typeinfo(sc)## ans = sq_string
## ans = scalarThis comes in use when displaying numbers, which we will get onto next:
Strings can be displayed by:
disp() functionprintf() (this prints to screen)sprintf() (this creates a string variable)fprintf() (this writes to a file or to the command window)This displays the values as ‘answers’ and on separate lines, and it makes no difference whether a value is a string or not:
'Normal body temp is'
36.5
'to '
37.5
'°C'## ans = Normal body temp is
## ans = 36.500
## ans = to
## ans = 37.500
## ans = °CThe disp() function will display one or more strings together on the same line. Remember that numbers have to be converted to strings for this to work:
body_temp_low = 36.5; % °C
body_temp_high = 37.5; % °C
disp(['Normal body temp is ', num2str(body_temp_low), ' to ', num2str(body_temp_high), ' °C'])## Normal body temp is 36.5 to 37.5 °CThe printf() function and its relatives (sprintf() and fprintf()) create strings from other strings and from numbers, but they allow you to format how they look using format specifiers inside strings. There are five main specifiers: %s, %d, %e, %f and %g.
The %s specifier inside a string will insert a different string into that location. The special character \n will create a newline, so remember to include that at the end of your formatted string to ensure your output doesn’t look weird:
% Insert a string into a string
printf('Insert a string here: %s\n', "Here's the string to insert")## Insert a string here: Here's the string to insertThe %Ws specifier will insert a string and, if the string is less than W characters wide, it will add whitespace ‘padding’ to make up the difference. If the string’s width is more than W characters, it has no effect:
% Insert a string with padding
printf('Insert a string here: %10s\n', 'String')
% Insert a string with padding that doesn't get used
printf('Insert a string here: %5s\n', 'String')## Insert a string here: String
## Insert a string here: StringThe %d specifier will insert a number (d is for decimal) and %Wd will add padding if the number is less than W characters wide. If you want to insert a percent sign, you need to use “%%” where the first is an ‘escape character’ that tells Octave you want to literally insert a percent sign:
test_score = 78;
max_score = 90;
percent = test_score / max_score * 100;
% Insert numbers into a string
printf('Your test score was: %d/%d (%d%%)\n', test_score, max_score, percent)
% Insert numbers into a string with padding
printf('\nScore Max %%\n%5d%5d%9d\n', test_score, max_score, percent)## Your test score was: 78/90 (86.6667%)
##
## Score Max %
## 78 90 86.6667The %e specifier will insert a number in scientific notation (ie with an exponential), %f will insert a fixed point number and %g will insert a number in whichever format Octave thinks is good:
c = 299792458; % m/s
printf('The speed of light is: %e m/s\n', c)
printf('π = %f\n', pi)
width = 0.000075; % m
printf('The width of a human hair is about %g m\n', width)## The speed of light is: 2.997925e+08 m/s
## π = 3.141593
## The width of a human hair is about 7.5e-05 mBuilding on the above, the %W.De specifier will insert a number in scientific notation with a minimum width of W (including the decimal point, a possible minus sign and five for the exponent) and D digits after the decimal points. %W.Df will do the same for a fixed point number and %W.Dg will choose the better of %W.De and %W.Df.
c = 299792458; % m/s
printf('The speed of light is: %.3e m/s\n', c)
printf('π = %.3f\n', pi)
width = 0.000075; % m
printf('The width of a human hair is about %.6g m\n', width)## The speed of light is: 2.998e+08 m/s
## π = 3.142
## The width of a human hair is about 7.5e-05 mThe printf() function prints formatted strings to the terminal screen. The sprintf() function is identical except it returns formatted strings which you can then assign to variables and use elsewhere in your code. All the specifiers are exactly the same.
Here’s how to use formatted strings to create filepaths:
folderpath = '/home/rowannicholls/Downloads';
filename = 'strings.html';
% Join two strings together, return the output and assign it to a variable
filepath = sprintf('%s/%s', folderpath, filename);
% Display the formatted string
disp(filepath)## /home/rowannicholls/Downloads/strings.htmlNote that if you don’t assign the output of sprintf() to anything and omit the semi-colon, then it is identical to printf() except it prints ans = at the front:
folderpath = '/home/rowannicholls/Downloads';
filename = 'strings.html';
% Join two strings together, return the output and assign it to a variable
sprintf('%s/%s', folderpath, filename)## ans = /home/rowannicholls/Downloads/strings.htmlfprintf() is identical to printf() and sprintf() except the output is written to a file as oppose to the terminal or to a variable:
c = 299792458; % m/s
% Write to file
file = fopen('Output/example_string.txt', 'w');
fprintf(file, 'The speed of light is: %.3e m/s\n', c)A file example_string.txt gets created with the following content:
The speed of light is: 2.998e+08 m/sNote that if the file is omitted - or incorrectly created - then fprintf() is completely identical to printf():
c = 299792458; % m/s
% Print to screen
printf('The speed of light is: %.3e m/s\n', c)
fprintf('The speed of light is: %.3e m/s\n', c)## The speed of light is: 2.998e+08 m/s
## The speed of light is: 2.998e+08 m/s